There is an ever-increasing need in the electronics industry for low profile and economical means for interconnecting electrical circuits. Traditionally, interconnecting means have required the use of housings, contact terminals, and in many cases, solder. Elastomeric connectors are also used, particularly for connecting components such as liquid crystal displays (LCDs). The present invention eliminates the need for housings, contact terminals, and solder for many applications. The invention also eliminates the need for elastomeric connectors and their associated pressure sustaining assemblies.
A large number of printed circuit boards, membrane switches, and the like are used by the electronic industry. Often the circuitry is screen printed onto the desired substrate. Screen printing is safer and simpler than etching or other printing techniques. Furthermore, it is more easily automated and more economical than other processes, particularly when used to print on flexible substrates.
The invention as disclosed herein provides a means for mechanically and electrically interconnecting conductive circuit traces or areas that have been screened on one substrate with desired conductive circuit traces or areas on a second substrate. It is not necessary that the conductive circuit traces or areas on the second substrate be screened thereon.
Means for interconnecting conductive circuit traces or areas on two substrates where neither substrate has screened circuitry is disclosed in our copending U.S. patent application Adhesive Electrical Interconnecting Means Ser. No. 657,717, filed Oct. 4, 1984.
Typical screenable conductive inks are comprised of finely divided particles of conductive materials having a major axis of approximately 15 micrometers in diameter and a minor axis of approximately 3 micrometers dispersed in a polymer medium comprised of a resin binder and solvent. In accordance with the invention, the basic conductive ink composition is modified by adding from about 1 percent to about 50 percent of large essentially spheroidal conductive filler particles, said particles ranging in mean size from about 15 micrometers to about 90 micrometers. Additional resin binder and solvent are also added to essentially preserve the resin-to-conductive-solids ratio of the ink base and screenable properties of the formulation. The large particles are randomly scattered throughout the medium as both individual and agglomerated particles. The resulting conductors are comprised of the polymer medium having a first group of finely divided particles and a second group of both individual and agglomerated large particles. The large particles are sufficiently large so that they will protrude from the surface of the modified conductive ink after it has been deposited on a substrate.
In accordance with the invention, the interconnecting means is made by screen printing and modified ink composition in the desired pattern onto a substrate. A layer of thermoplastic or heat activated adhesive is then deposited over the surface of the substrate and in particularly over the exposed surfaces of the inked areas which will be interconnected to conductive areas on another substrate. Interconnection is effected by positioning the desired areas in an overlapping conducting relationship such that the thermoplastic layer is disposed between the two conductive areas. Heat and pressure are applied to the positioned areas thus causing the thermoplastic layer to soften and flow from the positioned areas, and expose the protruding conductive particles or particle clusters which then interconnect said aligned conductive areas. Concomitantly, the surrounding areas of substrate are bonded by the adhesive.
The modified ink composition in accordance with the invention can be used on a variety of substrates, both firm and flexible. The composition can be screened in a variety of patterns. In one embodiment, the interconnection means is comprised of a continuous strip of flexible film having a plurality of parallel elongated modified conductive ink traces deposited thereon, the surfaces of said traces being essentially covered by dielectric thermoplastic adhesive to form a flexible cable connector. The flexible cable connector can be cut to length and adhered to a second substrate anywhere along its length since the interconnecting capability is built into the traces themselves.
The invention as disclosed herein is particularly useful for point-to-point matrix interconnections. Such interconnections can be made to any exposed conductive traces or areas or with other conductive areas having the ink traces in accordance with the invention.
Interconnecting means made in accordance with this invention offer connecting capabilities within the circuit path or area itself thus permitting direct connections between two surfaces without need for a housing. This is particularly advantageous when on glass with other non-crimpable surfaces. Furthermore, these interconnection means are particularly suitable for automatic assembly processes. In accordance with this invention these interconnecting means offer discrete and mass termination capabilities. If desired, the circuits can be terminated with standard connectors.
The interconnecting means disclosed herein can also be used for surface mounting electrical components to a substrate. The means is especially suitable for mounting leadless components to either flexible or firm substrates.
The use of the modified conductive ink formulation and interconnecting means can be understood by referring to the following drawings.